1. Field of the Invention
The present invention concerns a method of acquiring and processing seismic data used in the context of repetitive monitoring of the displacement of fluids impregnating a reservoir deep in the earth below the surface weathered zone.
The invention has a particularly advantageous application in the field of applied geophysics including the collection of information on the displacement of hydrocarbons and water in an underground reservoir by repeated seismic reflection methods.
2. Description of the Prior Art
Seismic reflection enables deformation of the earth to be reconstituted by measuring the travel times of seismic waves reflected at the main interfaces. If the signal to noise ratio of the recorded data is high it is possible to measure the reflectivity of these interfaces which is proportional to the amplitude of the reflected seismic waves. It is thus possible to monitor variations in the lithology or porosity of the sub-surface and therefore to define more precisely the nature of underground hydrocarbon reservoirs, for example.
With reference to detecting fluids in such a reservoir, it so happens that under favorable conditions, meaning an unconsolidated and porous reservoir containing gas or low viscosity oil, the presence of fluids or the position of their interfaces causes amplitude anomalies or changes in configuration in the seismic reflections evidenced on a seismic profile. However, direct detection of fluids in this way is usually not possible because of the low variations in reflectivity resulting from the displacement of the fluids or because the method employed has a low resolution.
One technical solution to the above limitations of simple reflection seismic methods is repetitive monitoring, i.e. a seismic reflection recording process that is repeated at time intervals sufficient for the displacement of the fluids in the reservoir to produce detectable differences in the travel time or amplitude of the reflected waves between the successive seismic reflection profiles. By comparing two sets of recorded data the relative measurement of the reflected seismic waves is much more sensitive than a single data acquisition in the case of one-off reflection seismic methods because it shows up the displacements of the fluids without the latter being detected directly.
However, regardless of the method employed for recording the seismic reflection data, repetitive monitoring requires the repeated use of surveying devices that have to be replaced exactly in their former position and the logged data must have a high signal to noise ratio and good vertical resolution.
Prior art seismic reflection methods currently used in the case of repetitive monitoring are 2D or 3D surface methods, or cross-well or combined surface-well methods such as the vertical seismic profile (VSP). For background information on geophysical methods employed for the repetitive monitoring of fluids see "New Dimensions in Geophysics for Reservoir Monitoring, SPE formation Evaluation", June 1991, p. 141-150 (R. J. Greaves, W. B. Beydoun and B. R. Spies).
Generally speaking, all the aforementioned seismic reflection data acquisition methods are directed to obtaining a continuous image of the sub-surface (over a surface, along a line or near a borehole) and all use a multiple coverage stacking method to improve the signal to noise ratio of the logged data.
None of these methods, although familiar to the man skilled in the art, is satisfactory in the context of repetitive monitoring as they lead, on land, to problems of accessibility and seasonal variations in the properties of the surface layers of the earth and, offshore, to problems with accurate positioning for the successive surveys.
On land, 2D or 3D surface reflection seismic surveys are often difficult because they require the installation of a large number of receivers over a large area. This leads to problems of accessibility to the terrain (viability, permits) and to the immobilization or need to reposition a large quantity of data acquisition equipment. Also, surface seismic data has a moderate signal to noise ratio because the seismic noise level is high at the surface. The surface seismic noise is made up of a ground wave and an air wave. The resolution of the data is low because high frequencies are attenuated in the surface weathered zone and this is combined with filtering and with interference between the receivers placed on the ground. Also, surface seismic methods are of relatively high cost.
Borehole seismic methods supply data whose resolution and signal to noise ratio are usually suited to repetitive monitoring. However, they are difficult to use because of the limited accessibility of the boreholes and such surveys are therefore restricted to localized areas around existing boreholes.
The present invention proposes a new method of acquiring and processing seismic data suitable for repetitive seismic reflection monitoring of fluids in an underground reservoir which is directed to alleviating the drawbacks of the previously mentioned prior art seismic methods.